Field of the Invention
The present invention generally relates to optical modules, and more particularly to an optical module capable of stabilizing the wavelengths of light output signals.
A wavelength division multiplexing system (hereinafter referred to as a WDM system) using the wavelength division multiplexing technique has an increased transmission capacity as the number of wavelengths to be multiplexed increases. Thus, it is required to reduce the intervals of the wavelengths of the light signals in order to increase the channel capacity. However, it is necessary to improve the precision of the wavelengths of the light signals output from an optical module In order to reduce the intervals of the wavelengths of the light signals.
A conventional optical module has a module construction designed to suppress a time-based variation in the wavelength of a laser diode source or a variation therein due to the peripheral temperature and to lock the wavelength of the light signal to be output. An example of an optical module having such a module construction is an optical module having a wavelength locking function of suppressing a wavelength variation of the optical signal. The wavelength locking function is implemented by, for example, a wavelength detection module called wavelength locker.
A description will now be given, with reference to FIGS. 1 and 2, of an optical module which does not have a built-in locker. FIG. 1 is a side view of such an optical module 1, and FIG. 2 is a top view thereof.
The optical module 1 includes a laser diode (LD) element 10, an LD carrier 11, a photodiode (PD) carrier 12, a monitor photodiode (PD) 13, a thermoelectric (TEC) element 14, a first lens 15, a thermistor (temperature sensing resistor) 16, a mount carrier 17, an optical isolator 18, and a second lens 19.
The LD element 10, which is a light-emitting element, is mounted on the LD carrier 11, and outputs light signals in the back and forth directions. The light signal emitted forward is collimated by the first lens 15 placed on the mount carrier 17, and is supplied to the optical isolator 18.
The optical isolator 18 allows the forward light signal supplied from the first lens 15 to pass therethrough, and shuts off the reflected light supplied from the second lens 19 in the backward direction. The light signal passing through the optical isolator 18 is focused by the second lens 19, and is supplied to the optical fiber 20.
The light signal output from the LD element 10 in the backward direction is monitored by the monitor PD 13 mounted on the PD carrier 12, and is utilized to perform automatic power control (APC) directed to controlling the forward light signal output at a constant level.
The above-mentioned LD carrier 11, the PD carrier 12 and the first lens 15 are mounted on the TEC element 14 via the mount carrier 17. The thermistor 16 is mounted on the mount carrier 17, and monitors the temperature in the vicinity of the LD element 10.
A description will now be given, with reference to FIGS. 3 and 4, of an optical module having a built-in wavelength locker. FIG. 3 is a side view of such an optical module 2, and FIG. 4 is a top view thereof. The optical module 2 is the same as the optical module 1 except for some parts, and parts that are the same as those shown in FIGS. 1 and 2 are given the same reference numbers,
The optical module 2 includes the LD element 10, the LD carrier 11, the PD carrier 12, the monitor PD 13, the TEC element 14, the first lens 15, the mount carrier 17, the optical isolator 18, the second lens 19, a back lens 21, a PD carrier 22, an optical filter 23, a beam splitter (BS) 24, and a monitor PD 25.
The light signal backward emitted from the LD element 10 is focused by the back lens 21 and is supplied to the beam splitter 24. Then, the beam splitter 24 reflects part of the received light signal, and allows the remaining part thereof to pass therethrough. Thus, the backward light signal is split into two lights. One of the two light beams thus split is monitored by the monitor PD 25 mounted on the PD carrier 22, and is utilized to perform the APC control directed to controlling the light signal output emitted forward to a constant level. The other of the two split-light signals is supplied, via the optical filter 23, to the monitor PD 13 mounted on the PD carrier 12.
The optical filter 23 used in the optical module 2 has a transmittance characteristic which is inclined to the wavelength of the light signal. For example, the optical filter 23 is an etalon filter (Fabry-Perot etalon filter), a lowpass filter, highpass filter, or a bandpass filter. A wavelength fixing control method of locking the wavelength of the light signal output from the LD element 10 is implemented using the outputs of the monitor PD 13 and the monitor PD 25.
FIG. 5 is a block diagram illustrating an example of the wavelength fixing control. The light signal backward emitted from the LD element 10 is partially reflected by a beam splatter 24-1, and is then supplied to the monitor PD 25. Of the light signal backward emitted from the LD element 10, the light signal passing through the beam splitter 24-1 is reflected by a beam splitter 24-2, and is supplied to the monitor PD 13 via a bandpass filter used as the optical filter 23. The monitor PDs 13 and 25 supply monitor currents as shown in FIG. 6 to a divider circuit 26, which will be described later. FIG. 6 is a graph of a monitor current vs. oscillation wavelength characteristic.
The monitor current output from the monitor PD 25 has a flat characteristic which does not depend on the frequency. The monitor current output from the monitor PD 13 indicates the performance of the optical filter 23 because the monitor PD 13 is supplied with the light signal via the optical filter 23.
For example, if it is wished to lock the oscillation wavelength at a wavelength xcex1 shown in FIG. 6, the oscillation wavelength of the LD element is set to xcex1 by utilizing the situation in which the oscillation wavelength of the LD element 10 varies due to the operation temperature thereof. Then, the divider circuit 26 is supplied with the monitor currents output from the PDs 13 and 25.
The divider circuit 26 performs a dividing operation on the values of the supplied monitor currents, and produces a resultant output signal as shown in FIG. 7, which shows an example of the output signal of the divider circuit 26.
As shown in FIG. 7, the output value of the divider circuit 26 increases or decreases when the oscillation wavelength deviates from xcex1. A temperature control circuit 27 shown in FIG. 5 controls the TEC element 14 in accordance with the value supplied form the divider circuit 26, and controls the temperature of the periphery of the LD element 10, whereby the oscillation wavelength of the LD element 10 can be adjusted.
However, the conventional optical module as shown in FIGS. 3 and 4 needs a large area for mounting the beam splitter 24 which splits the incident light into the two light components. Hence, the distances between the LD element 10 and the monitor PDs 13 and 25 increase, and the back lens 21 is required to compensate for the long distances. Thus, a large number of components is used to form the conventional optical module, which increases the cost.
Also, there is an increased number of points at which an optical adjustment such as an optical axis alignment is carried out. This increases the number of assembly steps.
Further, in order to increase the transmission or channel capacity, it is required to use a tunable LD element which can be tuned to a plurality of oscillation wavelengths in a single optical module and to improve the precision of the wavelengths of the light signals emitted from the tunable LD element.
It is a general object of the present invention to provide an optical module in which the above drawbacks are eliminated.
A more specific object of the present invention is to provide a less-expensive optical module assembled by a reduced number of components and capable of adjusting a plurality of oscillation frequencies with improved precision.
The above objects of the present invention are achieved by an optical module comprising: a first level detecting part having a semi-transparent structure and receiving a light signal emitted from a light-emitting element; a second level detecting part receiving the light signal which penetrates through the first level detecting part and passes through an optical filter; and a control part controlling an operation temperature of the light-emitting element on the basis of electric signals output from the first and second level detecting parts. Since the first level detecting part has the semi-transparent structure, the light signal penetrating through the first level detecting part can be applied to the second level detecting part without any optical components for splitting light. Hence, the optical module can be assembled by a reduced number of components at reduced cost.
The above objects of the present invention are also achieved by an optical module comprising: a light-emitting element capable of varying an oscillation wavelength based on an operation temperature; a splitter splitting a light signal emitted from the light-emitting element; a first level detecting part receiving a first split light signal from the splitter; a second level detecting part receiving a second split light signal from the splitter via an optical filter; a first control part controlling the operation temperature of the light-emitting element in accordance with electric signals output from the first and second level detecting parts; and a second control part controlling an operation temperature of the optical filter in accordance with the oscillation wavelength of the light-emitting element. The first and second control parts separately control the operation temperatures of the light-emitting element and the optical filter, respectively. Thus, the wavelength of the light signal emitted from the light-emitting element can be controlled precisely.
The above objects of the present invention are also achieved by an optical module comprising: a light-emitting element: a first temperature control part controlling a temperature of the light-emitting element; a first light receiving part receiving a light from the light-emitting part; a second light receiving part receiving the light from the light-emitting element via an optical filter; and a second temperature control part controlling a temperature of the optical filter. The first and second temperature control parts separately control the operation temperatures of the light-emitting element and the optical filter, respectively. Thus, the wavelength of the light signal emitted from the light-emitting element can be controlled precisely.
The above objects of the present invention are also achieved by an optical module comprising: a light-emitting element; a first light receiving part having a semi-transparent structure and receiving light from the light-emitting element; a second light receiving part receiving the light penetrating through the first light receiving part and passing through an optical filter: and a temperature control part controlling a temperature of the light-emitting element. Since the first light receiving part has the semi-transparent structure, the light signal penetrating through the first light receiving part can be applied to the second light receiving part without any optical components for splitting light. Hence, the optical module can be assembled by a reduced number of components at reduced cost. In addition, the operation temperature of the light-emitting element can be controlled, so that the wavelength of the light can easily be adjusted.